BENTHIC FORAMINIFERAL EFFECT ON NITROGEN AND CARBON CYCLING

Abstract

Abundances of fossil planktonic and benthic foraminifera are widely used to reconstruct marine paleoenvironments. Although poorly quantified, occurrences of benthic species are partly determined by in-sediment redox conditions. Understanding the relationship between redox zonation and species-specific microhabitat selection will improve the applicability of foraminifera in reconstructing benthic habitats. By conducting protozoan-free microcosm-scale experiments under controlled conditions, we quantified changes in redox zonation after killing the meiofauna and after artificially increasing foraminiferal abundances. During these experiments, we also quantified the effect of foraminiferal abundance on organic matter mineralization rates in soft sediments typical of nearshore environments. Results show that presence or absence of a meiofauna did not affect carbon mineralization rates. However, oxygen penetration depths were considerably shallower in microcosms where the meiofauna was killed, most likely due to the absence of meiofaunal bio-irrigation. Additionally, the distinctly larger nitrate fluxes observed in these microcosms demonstrate a rapid initial release of nitrate (15 days), followed by increased nitrification and denitrification activity, indicating that meiofaunal tissue can serve as a source of labile organic matter for bacterial mineralization. Similar foraminiferal abundances and depth distributions in the microcosms with natural and increased bacterial numbers (i.e., addition of bacterial food for the meiofauna) indicate that foraminiferal grazing rates on bacteria are low and do not affect redox chemistry considerably.